The present invention relates generally to integral power steering apparatuses, and more particularly to an integral power steering apparatus including a stroke limiter for limiting the stroke of a piston thereof.
In general, an integral power steering apparatus includes a hydraulic cylinder, and a piston slidably fitted within the hydraulic cylinder. The piston and a piston seal attached to the outer peripheral wall of the piston divides the inner space of the hydraulic cylinder, to define two hydraulic chambers. A control valve directs and supplies hydraulic fluid to each of the two hydraulic chambers. The difference between the pressures in the hydraulic chambers applies force to the piston to travel and thereby produces assist torque in a steering direction. Some integral power steering apparatuses include a stroke limiter, which relieves steering assist torque during steer angle being out of a desired steer angle range, to prevent a steering linkage assembly to be imposed with great load. Some stroke limiters relieve pressurized hydraulic fluid from one of the hydraulic chambers to the other hydraulic chamber via a check valve provided inside the piston. However, pistons for power steering systems are carburized in general. Therefore, providing a check valve within a piston needs machining after carburization. In addition, locating a check valve within a piston is under considerable constraints, especially under the constraint on angular position in the outer peripheral wall of the piston. Furthermore, a power steering apparatus with a stroke limiter and a power steering apparatus with no stroke limiter include different elements, which results in cost-up in management of parts. Accordingly, an integral power steering apparatus in which internal parts such as a piston are employed with no change from those of an integral steering apparatus with no stroke limiter is disclosed in Japanese Utility Model Registration No. 2518888 (hereinafter referred to as “JP2518888”), especially in pages 2 through 3, and FIG. 1 therein. The integral power steering apparatus disclosed in JP2518888 includes a hydraulic cylinder, a piston slidably fitted within the cylinder, a rack defined in the side wall of the piston, a sector gear meshed with the rack, a piston seal attached to the piston, the piston and the piston seal defining two hydraulic chambers, a control valve, a gear chamber being in fluid communication with one of the hydraulic chambers and accommodating the sector gear, and two valves provided in the wall of the gear chamber on both sides of the sector gear. In this structure, the state of the control valve is varied according to operations of steering wheel, to supply hydraulic pressure generated by an oil pump to one of the hydraulic chambers and to communicate the other hydraulic chamber and an oil reservoir at the same time, so that the difference between the pressures in the hydraulic chambers applies force to the piston to travel and thereby produces assist torque in a steered direction. One of the valves is disposed between the gear chamber and the hydraulic chamber in fluid communication with the gear chamber, and the other valve is disposed between the gear chamber and the oil pump. One of the valves is opened during the angular displacement of the sector gear being greater than a predetermined threshold angle in each direction.